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We're having the same problem here:
https://mundosecreter.com/foro/foro-principal/
Some firefox users can't type in most textareas. Refreshing the page sometimes seems to work, but not always.
It's probably related with some firefox plugin or something, because I can post (I have no plugins installed because I barely use it). It's quite annoying.
Exclude the wpforo page from WP Super cache.
This FAQ should be helpful for you: https://wpforo.com/community/faq/wpforo-and-cache-plugins/
Sadly, this didn't work. Any other ideas?